Medical equipment herein termed "patient breathing apparatus" is used to deliver oxygen with anesthetic gases and vapours ("breathing gas") to a patient while surgical procedures are performed on the patient. Usually, such apparatus provides a mixture of oxygen, with or without nitrous oxide, for inspiration by the patient, and may conventionally include controls for either assisting or controlling breathing, exhaled volume indicators, alarm systems, positive end expiratory pressure ("PEEP") valves, pressure indicators, gas concentration monitors, flow indicators, heated humidifiers for warming and humidifying the breathing gas and tubing for interconnecting these components with each other and with the patient, thereby forming a "patient breathing circuit". Often, the anesthetist who monitors and controls the apparatus must add specific ancilliary devices and accessories to the patient breathing circuit, as warranted by factors such as the physiological status of the patient, the nature of the surgical procedure, the anesthetic technique employed, etc.
Traditionally, patient breathing circuits have adopted the so-called "circle" configuration in which breathing gas delivered to the patient is substantially constrained, by a pair of one-way valves, to flow in one direction around a circuitous path. The gas flows through an inspiratory hose or "channel", into the patient's lungs and is then returned, after expiry by the patient, through an expiratory channel (another hose), to a carbon dioxide absorber which removes carbon dioxide from the expired gas so that it may be re-cycled to the patient via the inspiratory channel. Periodically, (or continuously at low gas flow rates) fresh anesthetic or other gases may be introduced into the inspiratory channel, and some portion of the expired gas may be allowed to escape from the expiratory channel for collection by a scavenging device, rather than being returned to the patient.
Most anesthetic machines currently marketed are supplied by the manufacturer with fittings specifically adapted to the "circle" circuit configuration. However, the circle circuit configuration is not universally employed. For reasons hereinafter discussed in greater detail, anesthetists often change the configuration of anesthetic machines from the "circle" circuit to the so-called "Bain circuit" configuration.
The primary characteristic distinguishing the Bain circuit configuration from the circle circuit configuration is that, in the Bain circuit, the inspiratory and expiratory breathing hoses are coaxial, whereas they are series-connected in the circle circuit. In a typical Bain circuit, fresh gas is delivered through the innermost co-axial hose to the distal end of that hose, for inspiration by the patient. The outer, larger diameter hose, which surrounds the fresh gas delivery hose, serves as a fresh gas reservoir containing some fresh gas for inspiration by the patient, and as an expiratory channel for communicating gas exhaled by the patient to the exhaust/scavenging portion of the circuit. Further, in the Bain circuit, unlike the circle circuit, breathing gas does not flow unidirectionally around the circuit. Rather, the direction of gas flow in the Bain circuit reverses periodically as the patient inspires and expires breathing gas.
Heretofore, anesthetic machines supplied with fittings adapted to the circle circuit configuration have not been readily convertible to the Bain circuit configuration. Such conversion has heretofore required an expensive Bain circuit mount and has necessitated duplication of several circuit components (i.e. exhaust valves, connections to ventilators and to scavenging systems). The conversion procedure results in a profusion of tubing which is cumbersome and time consuming to interconnect. Further, the conversion procedure heretofore adopted is of such complexity that there is a significant potential that an error may be made during conversion from one configuration to another, with an attendant risk of patient injury.
The present invention provides an adaptor which facilitates rapid conversion of patient breathing apparatus between the circle and Bain circuit configuration. Such conversion obviates the need for an expensive Bain circuit mount, is accomplished without requiring expensive duplication of circuit components, and significantly reduces the complexity of the conversion operation, thereby minimizing the likelihood that a deficient patient breathing circuit may be configured, and accordingly minimizing the risk of patient injury.